There is a seller on ebay who is selling 'FG' steel toroidal cores for 7.6$ a pound.
Guaranteed max core loss at 60Hz:
0.970 W/Kg @ 1.5 Tesla
1.39 W/Kg @ 1.8 Tesla
Saturation flux density: 2.035 Tesla ( 20350 Gauss)
the steel sold by alpha cores direct is 1.7T saturating at 1.95T iirc. 1 watt per kilogram losses at 60hz @1.7T.
steel that saturates at 1.5T should be far, far cheaper than 7$ per kilogram or $3.5/lb, boB i think that stuff is designed for saturatable reactors, not transformers.
whoever is selling those FG steel cores on ebay is probably making a killing, though not too many have been sold.
there are not many other options for laminations, though there are companies that offer custom water jet cutting of non grain aligned steel for prototype motors.
the folks at diy audio have been complaining as well for some time about hard to get EI laminations of suitable quality and not by the ton either, someone there has a source but i think he is in south america IIRC and most likely pays significantly more than $3/lb for them.
another option perhaps is amorphous metal.--the same metal many utilities are using in their distribution transformers. almost zero losses but you'll need to wait 40 years before it pays for itself.
another option is buy a 20 amp variac and rewind it
http://www.ebay.com/itm/20-AMP-110V-VARIAC-AUTOTRANSFORMER-VOLTAGE-REGULATOR-POWERSTAT-0-130V-OUTPUT-NEW-/381222191973i find it hard to believe this would be cheaper than buying a 2KW toroid core, but nothing surprises me anymore.
my "500" watt variac is 6 watts no load probably made by the same folks as ^that ebay link